As ethane recovery becomes increasingly economically attractive, various configurations have been developed to improve the recovery of ethane from natural gas liquids (NGL). Most commonly, numerous processes employ either cooling of feed gases via turbo expansion or a subcooled absorption process to enhance ethane and/or propane recovery.
For example, a typical configuration that employs turbo expansion cooling assisted by external propane and ethane refrigeration is shown in Prior Art FIG. 1. Here, the feed gas stream 1 is split into two streams (2 and 3) for chilling. Stream 3 is cooled by the demethanizer side reboiler system 111 to stream 24, while stream 2 is chilled by the cold residue gas from separator 106 and demethanizer 110 (via streams 13, 18, and 38). The two streams 2 and 3 are typically chilled to about −102° F., and about 15% of the feed gas volume is condensed. The liquid condensate volume is about 3800 GPM (at a typical feed gas flow rate of 2 BSCFD supplied at about 600 psig and 68° F. with a composition of typically 1% N2, 0.9% CO2, 92.35% C1, 4.25% C2, 0.95% C3, 0.20% iC4, 0.25% nC4 and 0.1% C5+), which is fed to the upper section of the demethanizer 110 via lines 8 and 9 and JT valve 104. The vapor stream 7 is expanded via expander 105 and the resulting two-phase mixture from line 12 is separated in separator 106. Over 80% of the feed gas is flashed off as stream 13 in separator 106. Separated liquid 14 is pumped by pump 107 via line 15 to the demethanizer operating typically at 400 psia. The demethanizer produces a residue gas 18 that is partially depleted of ethane and an NGL product 23 containing the ethane plus components. Side reboilers 111 are used for stripping the methane component from the NGL (via lines 25-30) while providing a source of cooling for the feed gas 3. The demethanizer overhead vapor stream 18 typically at −129° F. combines with the flash gas stream 13 from separator 106 and fed to the feed exchanger 101 for feed gas cooling (Additional cooling is provided via external ethane and propane refrigerants via lines 44 and 45).
Unfortunately, such a process is typically limited to 60% ethane recovery and 94% propane recovery. Further reduction in demethanizer pressure produces marginal improvement in recoveries, which is normally not justified due to the higher cost of the residue compression. Moreover, at such conditions, the demethanizer will operate close to the CO2 freezing temperature.
Another known configuration for ethane recovery is a gas subcooled process as shown in Prior Art FIG. 2, which typically employs two columns, an absorber and a demethanizer and a rectifier exchanger to improve the NGL recovery. In a typical design, the feed gas is cooled in feed exchanger 101 to −85° F. with refrigeration supplied by residue gas 38, side reboilers stream 25 and stream 27, propane refrigeration 44 and ethane refrigeration 45. About 5% of the feed gas is separated in separator 103, producing 1100 GPM liquid (with feed gas parameters similar or substantially identical as described above) which is further letdown in pressure and fed to lower section of absorber 108. Vapor stream 7 from the separator is split into two streams that are individually fed to the rectifier exchanger and the expander. About 66% of the total flow is expanded via expander 105 and fed to the middle section of absorber 108 and the remaining 34% is cooled in a rectifier exchanger 109 to −117° F. by the absorber overhead vapor. The exit liquid from exchanger 109 is letdown in pressure to 390 psia while being cooled to −137° F. and routed to the top of the absorber as reflux. The absorber generates a residue gas at −138° and a bottom intermediate product at −118° F. that is pumped by pump 112 and fed to the top of demethanizer 110. The demethanizer produces an overhead gas 22 that is routed to the bottom Of the absorber and an NGL product stream 23 containing the ethane plus components. Side reboilers are used for stripping the methane component from the NGL while providing a source of cooling for the feed gas. The absorber overhead vapor stream 18 typically at −138° F. is used for feed cooling in the rectifier exchanger 108 and feed exchanger 101.
However, such configurations are frequently limited to 72% ethane recovery and 94% propane recovery. Similar to the previous known configurations of Prior Art FIG. 1, further reduction in demethanizer pressure produces marginal benefit in recoveries, which is normally not justified due to the higher residue compression requirement.
Thus, although various configurations and methods for relatively high ethane recovery from natural gas liquids are known in the art, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need for improved configurations and methods for high ethane recovery, and especially where the feed gas has a relatively low pressure.